If you've got the circlips off (vital!) and the joints still aren't releasing, grab your propane torch and heat the yoke ears up to 400-500*F (per either a Tempil(tm) stick or non-contact thermometer.) That's well below the point where you'll wreck the temper of the shaft, but high enough that you'll relieve the press fit and should be able to pop the joint loose.
I find it handy to keep a small assortment (typically 400*, 500*,600*, 700*) of Tempil sticks handy - they look like crayons, but you can make a mark on a part you're heating up, and it will melt withing 95-105% of the rated temperature value. Find Tempil(tm) sticks at most welding supply houses.
Oh - and heat only the yoke years! Do not heat the joint proper! If you heat the joint, you're going to have trouble, since it's going to expand and force the bore open (which, essentially, tightens the press-fit.)
You can also heat up the yoke ears, and then touch a block of paraffin wax (or just a candle) to the junction between the cap and the yoke. Let the wax melt and wick in (you won't need the torch on it,) and let it cool a bit - the wax usually gets in there and helps lubricate things to free them up.
Paint the caps on the new Cardan joint with never-seez to make it easier to take to bits next time...